Summary:
Forrestania’s camp of nickel sulphide deposits is hosted by a sequence of ~2.9Ga meta-igneous and metasedimentary rocks that are part of the Forrestania Greenstone Belt (FGB) of the Youanmi Terrane of the Eastern Yilgarn Craton.
Forrestania’s nickel deposits is the ‘Kambalda-style’ Type 1 and Type 2 models where nickel sulphides are interpreted to have accumulated in channelised komatiitic lava flows. Many of Forrestania’s deposits are often tectonically displaced from the original depositional locations and have been deformed and/or dismembered by one or more local and/or regional post-depositional tectonic events.
The main deposit type is the komatiite hosted, disseminated to massive nickel sulphide deposits, which include the Flying Fox and Spotted Quoll deposits. The mineralisation occurs in association with the basal section of high MgO cumulate ultramafic rocks.
The greenstone succession in the district also hosts several orogenic lode gold deposits, of which the Bounty Gold Mine is the largest example.
The Forrestania Greenstone Belt comprises two main lithological associations; a lower sequence of basalt-ultramafic Banded Iron Formation (BIF) ± metasediments, and an upper sequence of predominately finely laminated siltstones, shales and felsic metasediments. Up to six belts of ultramafic rock-types are recognised in the lower association, and the strike length of individual belts ranges from 20 to 90km. The ultramafic belts comprise komatiite sequences that show a wide variety of volcanic flow facies environments, including thick sequences of olivine adcumulate to mesocumulate hosted nickel deposits (Eastern Ultramafic Belt; hosting the Fireball, Diggers and Cosmic Boy deposits), channelised flow sequences with bounding flanking flow facies (Western Ultramafic Belt; hosting the Flying Fox, Spotted Quoll, New Morning/Daybreak and Willy Willy deposits and prospects), and thin spinifextextured flow units (Eastern Ultramafic Belt; hosting the Hang Dog and Emu Heights prospects). The ultramafic belts have mostly steep dips, some of which are locally overturned such as the Eastern Ultramafic Belt south of the Purple Haze Prospect. Five of the six ultramafic belts face west, with only the western belt facing east. Nickel deposits and occurrences are restricted to the Eastern and Western Ultramafic belts.
Flying Fox deposit
The deposit is mainly located along the traditional footwall of the basal ultramafic metasediment contact, which was the original locus for sulphide deposition from an overlying pile of komatiite flows. Subsequent metamorphism, deformation, and intrusion of granitoid sills have contributed to a complex setting, with mineralisation now occupying a possible shear zone.
The Flying Fox deposit strikes at 30° and dips nominally 65° east.
Dimensions
The strike length of the Flying Fox deposit varies considerably but is up to 750m in the T5 deposit. Distance from the top of T4 to the base of T5 is approximately 550m. The mean width of the deposit is 2.2m.
Spotted Quoll deposit
Spotted Quoll is located within the traditional footwall of the basal ultramafic metasediment contact, which was probably the original locus for sulphide deposition from an overlying pile of komatiite flows. Subsequent metamorphism, deformation, and intrusion of granitoid sills has contributed to a complex setting, with mineralisation now occupying a possible shear zone within the footwall sediments, 15 to 20m (stratigraphical) beneath the basalt/ultramafic contact.
The deposit is principally a body of matrix magmatic sulphide mineralisation in which the original pentlandite and pyrrhotite assemblage has been overprinted by arsenic-bearing assemblages dominated by gersdorffite and minor nickeline. Sulphide abundances of 20% to 90% are common.
Mean nickel grades of ore intersections are in the order of 4% to 12% Ni.
Dimensions
The strike length of the MRE is nominally 300m on average, with a range of 25m to 520m, depending on depth below surface. The nominal mean dip length is 1,500m.
The elevation below the pre-existing open pit is 1250mRL and the maximum depth of the Mineral Resource is 250mRL.
The mean thickness of the mineralised zone is 3.1m, with a maximum thickness of 13.4m.